1. Field of the Invention
This invention relates to a radio receiver for receiving a main radio broadcast signal having a particular transmission frequency and comprising a data signal containing information on one or more alternative frequencies to which the receiver may be tuned in case the quality of the main radio broadcast signal drops below predetermined standards, said receiver having a tuning section for selecting, amplifying and demodulating the main radio broadcast signal, a data signal demodulator for extracting and demodulating the data signal contained therein and, a microcontroller for extracting alternative-frequency information from the demodulated data signal and for communicating such alternative frequency information, through a serial communication bus to storage means of the tuning section.
2. Description of the Related Art
Receivers of this kind are, for instance, known from Application Note 96010 entitled: "User Manual of High End RDS/EON Car Radio System CCR525(V1.4)" and published by Philips Electronics Semiconductor Division. Herein, "RDS" stands for "Radio Data System", a digital data transmission system, which is transmitted simultaneously with radio broadcast signals and which gives to the receiver useful information on the radio broadcast transmission. RDS is nowadays widely used in European countries and more particulars thereof may be found in the "Specification of the radio data system RDS for VHF/FM sound broadcasting" Ref. nr. prEN 50067:1992 E, issued by CENELEC, which has been herein incorporated by reference. In the United States, a similar system, mentioned RDBS, is adopted, for which reference is made to the "Specification of the radio broadcast system RBDS" issued by NRSC on January, 1993.
One major service provided by RDS is the transmission of alternative frequency information, which is transmitted in the "alternative frequency list" of the RDS-signal of the main radio broadcast signal. This service gives the possibility to automatically switch the receiver to an alternative frequency transmission having the same program when the signal-quality of the main radio broadcast signal deteriorates. To properly carry out this function, the receiver must continuously monitor the quality of the alternative frequency transmissions. The quality check can, e.g., be a combination of measurements on RF-level, adjacent channel interference, multipath distortion, value of intermediate frequency, etc. Some quality measurements, such as on the RF level, can be done in a fraction of a millisecond, whereas others, for example, on multipath distortion, may require a few milliseconds for a stable result. These quality checks are carried out with the audio output muted to avoid aggressive audio plops and breakthroughs. But muting the signal leaves an audio gap which can also be annoying, especially if the gaps are at regular intervals.
To make the audio gap less annoying, most high-end receivers utilize a pause detector to update the quality record of the alternative frequency transmissions only in the quiet passages. This, however, limits the updating speed and may not be permitted when the signal conditions suddenly deteriorate.
One other possibility is to reduce the muting periods until the audio gaps in speech or music can no longer be perceived. Studies carried out on the audibility of such gaps have shown that gaps of up to 5 ms with gentle slopes of 1 ms are almost completely inaudible whereas gaps larger than 10 ms are in all cases completely audible. The problem with reducing the audio gaps to below 5 ms, with conventional tuning systems and updating algorithms, is that the quality measurement period would become very short. A typical RDS update requires about 1 ms for communicating the alternative frequency from the microprocessor through the serial communication bus to the tuning section, at least 1 ms for the fastest available phase-locked loop (PLL) to jump to the alternative frequency, at least 3 ms for the quality measurements, again 1 ms to communicate the original tuning frequency to the PLL and another 1 ms for the PLL to jump back to this original frequency, resulting in an audio gap of more than 7 ms.